This invention relates a trouble diagnosis device and method for an exhaust gas return control system which controls an operation of returning part of the exhaust gas of an internal combustion engine (hereinafter referred to merely as "an engine", when applicable) to the intake pipe of the latter (hereinafter referred to as "exhaust gas return" or "EGR", when applicable).
An EGR control device is generally provided for an engine to reduce injurious components such as NO.sub.x in the exhaust gas. An exhaust pressure control type EGR control device using an exhaust pressure transducer is popularly employed.
A trouble diagnosis device for an EGR control device has been disclosed, for instance, by Japanese Patent Application (OPI) No. 51746/1987 (the term "OPI" as used herein means an "unexamined published application"). The trouble diagnosis device operates as follows: When the engine is in steady operation, and an EGR control valve is open, the latter is temporarily closed, to suspend the exhaust gas returning operation. Under this condition, an operating condition of the engine is detected and stored as a detection value. The detection value thus stored is compared with the one detected before the EGR control valve is closed, and the result of comparison is utilized for detection of a trouble in the EGR control device.
The conventional trouble diagnosis device is designed as described above. Hence, when the amount of variation in the opening degree of the throttle valve exceeds a predetermined value, the trouble diagnosis operations is suspended, because it will greatly adversely affect the operation of the engine. However, the conventional trouble diagnosis device is still disadvantageous in the following point: That is, if, although the amount of variation in the opening degree of the throttle valve does not exceed the predetermined value, the variation rate in the opening degree of the throttle valve per unit time exceeds a predetermined value, then the trouble in the EGR control device is erroneously detected for instance because detection of the detection value of the engine operating condition is delayed.